The present invention relates to a method of injection-molding a thermoplastic resin, which permits the facile formation of a coating having various functions on the surface of an injection-molded article formed of the thermoplastic resin.
A coating is sometimes formed on the surface of an injection-molded article of a thermoplastic resin for improving the injection-molded article in surface properties. The coating refers, for example, to a coating of a coating composition, a hard-coating, an ultraviolet light-preventing coating or a non-fogging coating. Generally, after an injection-molded article is formed by an injection molding method, there is employed a separate step of forming a coating having various functions on the surface of the injection-molded article. The coating is formed, for example, by spraying a coating material on the molded article or by immersing the molded article in a coating material in a liquid state. As a result, the process for producing an end product whose surface is coated includes additional steps. The above injection-molded article, therefore, has the following problem to overcome. It is required to decrease the number of steps required up to the production of the end product, to decrease the scale of its production facilities, to decrease the processing and treating time and to decrease the production cost.
For compression-molding or injection-molding thermosetting resins such as SMC (sheet molding compound) and BMC (bulk molding compound), there are proposed some methods in which a coating is formed on a product during the molding step. For example, JP-B-55-9291 (corresponding U.S. Pat. No. 4,076,788) discloses a method in which an SMC material is fed into a space between an upper mold member and a lower mold member, these mold members are closed to compression-mold the SMC material, the resultant molded article is spaced from the upper mold member to form a space while the mold members are tightly closed, and a coating material in an amount smaller than the volume of the formed space is introduced into the space formed between the upper mold member and the molded article. In the present specification, the method of introducing a coating material in an amount smaller than the volume of a space formed between a molded article of a resin and a cavity wall will be referred to as "coating material short shot method" hereinafter.
JP-B-4-33252 (corresponding to U.S. Pat. No. 4,668,460) discloses a method in which an SMC material is fed into a space between an upper mold member and a lower mold member, these mold members are closed to compression-mold the SMC material, and a coating material is introduced into a boundary between the mold and the resultant molded article at a pressure greatly larger than the pressure generated between the mold and the molded article.
On the other hand, in the method of injection-molding a thermoplastic resin, there are proposed some methods in which a coating is formed on the surface of an injection-molded article during its molding step. For example, JP-A-5-301251 discloses a method for injecting a thermoplastic resin into a cavity formed in a mold and then introducing and filling a thermosetting coating material into a space formed of a coating surface of a molded article of the resin and the mold after the mold clamping force is decreased or while the mold clamping force is maintained. In the present specification, the method of introducing and filling a coating material in an amount equivalent to the volume of a space formed between a molded article of a resin and a mold will be referred to as "coating material full shot method" hereinafter.
Further, JP-A-5-318527 discloses a method in which a thermoplastic resin is injected and then an uncured thermosetting resin is introduced into a space formed by the contraction of the thermoplastic resin in a mold and then is cured, to form a molded article of the thermoplastic resin of which the surface is partly coated with the thermosetting resin.
The methods disclosed in JP-B-55-9291 and JPB-4-33252 are very effective for concealing the defects on a molded article surface such as pits and sink marks which are problems in the SMC compression molding. In the compression molding, the mold clamping force applied to a movable mold member is all exerted on a molded article. That is, the equation, ##EQU1## holds. The molding pressure can be therefore easily changed by controlling the mold clamping force, so that a space in the cavity into which a coating material is to be introduced can be uniformly and easily formed by controlling the mold clamping force.
In the method of injection-molding a thermoplastic resin, however, a series of molding steps are generally carried out without spacing a mold and in a state in which a predetermined mold clamping force is applied. The mold clamping force is exerted not directly on a molded article but on a mold. In these points, the method of injection-molding a thermoplastic resin greatly differs from the method of compression-molding a thermosetting resin. It is therefore difficult to apply the techniques disclosed in JP-B-55-9291 and JP-B-4-33252 to the method of injection-molding a thermoplastic resin.
That is, in the method of compression-molding a thermosetting resin such as SMC or the like, disclosed in JP-B-55-9291 and JP-B-4-33252, a compressive force (molding pressure) is always exerted on a molding material by a movable mold member all through the molding and processing steps. For introducing a coating material onto the molded article surface in the mold, therefore, it is required to form a space between the mold and the molding material by opening the mold and releasing the compressive force (molding pressure) caused by a movable mold member (JP-B-55-9291), or it is required to introduce a coating material at a pressure greater than the compressive force (molding pressure) caused by the movable mold member (JP-B-4-33252).
In the method of injection-molding a thermoplastic resin, generally, the mold is composed of a fixed mold member and a movable mold member. The fixed mold member is contact with the movable mold member, when the movable mold member is clamped to the fixed mold member before injecting a molten thermoplastic resin into a cavity of the mold. The molten resin is injected into the cavity from an injection cylinder to fill the cavity with the molten resin in a state in which the clamping force applied to the movable mold member is supported by the fixed mold member. The clamping force is applied to the movable mold member for preventing the movable mold member from opening a mold while or after the cavity is filled with the molten resin. That is, the clamping force itself is not a compressive force which is exerted on the molten resin injected into the cavity. In other words, the clamping force does not at all work directly to form (mold) an injection-molded article.
The above state can be expressed by the following equation. ##EQU2## in which P.sub.I is an injection pressure of a molten resin, P.sub.loss is a pressure loss at the time of injecting the molten resin, P.sub.comp is a compressive pressure which an excess of the filled molten resin in the cavity receives under the clamping force, and "projected area of molded article" is an area of the molded article when the molded article is projected to a plane perpendicular to the direction of the clamping force. In the present specification, the molding pressure refers to a pressure which is caused by a resin injected into the cavity and which is exerted on the cavity wall unless otherwise specified. The molding pressure can be measured on the cavity wall corresponding to any position of a molded article.
In the injection molding method, therefore, controlling the clamping force alone is not sufficient for controlling the molding pressure to a desired value. In other words, the space (gap) between the resin in the cavity and the cavity wall, into which a coating material is to be introduced, (such a space will be referred to as "the space" simply, hereinafter in some cases), cannot be uniformly or easily formed by controlling the clamping force alone.
When the technique of introducing a coating material at a pressure greater than the compressive force (molding pressure) caused by a movable mold member, disclosed in JP-B-4-33252, is applied to the technique of injection-molding a thermoplastic resin, it cannot be necessarily expected that the pressure for introducing a coating material is higher than the clamping force so that the coating material can be introduced into the boundary between a molded article and the cavity wall, due to (molding pressure).times.(projected area of molded article).ltoreq.clamping force.
In the techniques disclosed in JP-A-5-301251 and JP-A-5-318527, a thermoplastic resin injected into a cavity shrinks in its cooling and solidification process, thereby forming the space. A coating material is introduced and filled in the resultant space. In other words, the coating material in an amount exceeding the volume of the space is not always introduced into the space. The above techniques are seemingly proper as a technique of forming a coating in the method of injection-molding a thermoplastic resin. However, as already described, there is the relationship of ##EQU3## Meanwhile, for forming the space, the molding pressure is required to be 0 kgf/cm.sup.2. However, even if the cooling of molten resin in the cavity to solidness proceeds to attain P.sub.I =P.sub.loss =0, the molding pressure cannot be brought into 0 kgf/cm.sup.2 so long as the term of P.sub.comp remains. It is therefore not expected that the space is necessarily formed. Further, the formation of the space into which a coating material is to be introduced cannot uniformly or easily carried out by controlling the clamping force.
The molding pressure differs depending upon the form of the cavity, the kind of the molten resin used or the solidifying time (cooling time) of the injected molten resin. It therefore cannot be uniformly said that the space is reliably formed, for example, by spacing the movable mold member from the fixed mold member.
As explained above, the relation between the formation of the space and the decrease amount of the clamping force, or the relation between the formation of the space and the spacing amount of the movable mold member from the fixed mold member, depends upon the magnitude of the term of P.sub.comp. The formation of the space therefore cannot be uniformly or easily carried out by simply controlling the clamping force alone or by determining the amount of the spacing of the movable mold member from the fixed mold member.
Generally, the introduced coating material shrinks in volume. Concerning the molten resin in the cavity, dwell pressure is being applied to the injected molten resin from the injection cylinder side of an injection molding machine to replenish the molten resin into the cavity. Concerning the coating material, generally, after it is introduced into the space, the introduced coating material and the coating material introduction device are in a "relation-broken" state. That is, when the coating material is introduced into the space, pressure is not always exerted on the coating material introduced from the coating material introduction device. As a result, there arise problems in that the gloss of the surface of the coating decreases, that the adhesive ability of the coating to the thermoplastic resin decreases and that the coating is not uniform. For avoiding these problems, the introduced coating material is required to be always pressed to the cavity wall under a predetermined pressure. However, the above JP-A-5-301251 and JP-A-5-318527 are neither disclose nor suggest anything concerning these problems and means to solve them.